REDUCTION OF THE THRESHOLD FOR LONG-TERM POTENTIATION BY PRIOR THETA-FREQUENCY SYNAPTIC ACTIVITY

Activation of the lateral perforant path input to the dentate gyrus wi th theta-patterned conditioning trains produced LTP of synaptic effica cy that changed in magnitude as an inverted U-shaped function of the n umber of trains. The LTP induction function was not fixed, however, an d could be shifted to the left by administering 5 Hz ''priming'' stimu lation to the lateral path 10 min prior to the conditioning trains. Th e priming effect was input specific and selective to a narrow window o f stimulus frequencies. The shift to the left of the LTP induction fun ction by priming stimulation was blocked by the muscarinic receptor an tagonist atropine sulphate. Nimodipine, an antagonist of L-type voltag e-sensitive calcium channels, did not mimic the priming effect but ins tead produced a general facilitation of LTP induction. These data demo nstrate that the degree to which LTP is induced in the lateral path is a non-linear function of afferent activity, and that this function, i ncluding LTP threshold, can be shifted to the left by prior synaptic a ctivity at hippocampal theta-rhythm frequencies. (C) 1995 Wiley-Liss, Inc.